As a diagnostic method by an X-ray diagnostic apparatus, the K absorption edge subtraction method has been devised. The K absorption edge is a discontinuous point in the energy direction of X-ray photons which exist in an X-ray absorption characteristic of a substance. The energy at the K absorption edge of iodine used as a contrast agent is about 33.169 keV.
In the conventional K absorption edge subtraction method, an X-ray which has an energy slightly lower than the energy at a K absorption edge and an X-ray which has an energy slightly higher than the energy at the K absorption edge are emitted at a short time interval, using an X-ray source which generates highly monochromatic X-rays. Then, subtraction processing of two frames of X-ray image data acquired by exposing X-rays which have different energies is performed.
When an interval of imaging by exposing X-rays which have two energies is sufficiently short, compared to a moving speed of a human body, the human body can be considered as static. While the X-ray absorption characteristic of iodine is significantly different between energies lower and higher than the K absorption edge, the X-ray absorption characteristics of substances constituting a human body are approximately equal. Accordingly, the K absorption edge subtraction method allows removing the structure of a human body from image data.